The present invention relates to apparatus and methods for locating a short between two networks, such as a power and ground plane in a printed circuit board.
Modern printed circuit boards can be extremely difficult to troubleshoot. Even before being populated with electrical components, boards can be extremely complicated and have hundreds of feed-through holes connecting multiple circuit layers. These complex circuit boards can have a short circuit between two normally isolated, but rather extensive networks. For example, a short between a ground plane network and another extensive network such as the power lines, power planes or the reset lines can be very difficult to troubleshoot, given the large area over which a fault can occur.
When circuit components are installed in such a circuit board, the complexity increases and additional faults can occur from defective components, solder that may form a bridge between adjacent circuit traces etc. Accordingly, it can be easy to determine a short exists, but very difficult to locate such a short.
When a short occurs between the power and ground networks special difficulties exist. First, the circuit cannot be powered and conventional troubleshooting techniques cannot be applied. Normally, a troubleshooter may use a voltmeter and a oscilloscope to determine whether particular waveforms exist in a proper form at various stages in a circuit. Such troubleshooting is impossible when power cannot be applied to the circuit board. Often the troubleshooter who cannot power the circuit board, is reduced to searching for a short by eye, an obviously time consuming and probably unsuccessful undertaking. Furthermore, for multilayered boards, relevant planes may be buried and visual inspection becomes impossible.
A known method for locating a short circuit is to inject a small current (or a low voltage alternating current) through a conductor involved in a short. The direction of the current can be determined with a voltmeter by observing the voltage drop along the affected conductor. This known method employs a relatively small current, about 10 mA, to avoid overheating and damaging the affected conductors and the components connected to them.
These known techniques do not work well for important classes of printed circuit traces, such as the traces or planes for power and ground. Power and ground planes normally have relatively low resistance, in the order of one milliOhm. Conventional currents do not generate a voltage drop that can be easily measured and therefore tend to be masked by noise. For example, a conventional 10.0 mA current may be divided by alternate circuit paths to a 1.0 mA current flowing through one miliohm of resistance, producing only one microvolt, a rather difficult voltage drop to work with.
Furthermore, since ground planes do not offer a linear path to guide the search for a short, conventional linear tracing techniques do not apply well.
Known techniques for locating a short suggest moving a pair of probes, searching for an optimum response that occurs across the short. However, this technique assumes low impedance at the short. Actually the impedance at the short can vary by several orders of magnitude, making the response at the short unpredictable. When the conductors or planes of a very large network are involved, the number choices becomes so large as to be impossible to examine by the average trouble-shooter.
Furthermore, finding shorts between power and ground planes in boards populated with components is extremely difficult with conventional methods because direct current can validly pass from power to ground through various shunting components. For example, injecting alternating currents produces misleading indications because of the bypassing effect of various decoupling components.
A known apparatus for detecting a short (U.S. Pat. No. 4,565,966) measures capacitance at various nodes with respect to a reference network or reference plane. This patent also discusses measuring resistance at two points on each of two networks that are shorted together. This patent suggests that through calculation the resistances to the short can be determined, and therefore, the approximate distance to the short can be found. As before, a short having substantial and unpredictable. This patent also does not determine current direction and does not disclose the appropriate current magnitude.
U.S. Pat. No. 4,001,686 suggests that the magnitude and direction of a current can be measured by an undefined test instrument. The reference, however, does not concern itself with various details including the amount of current that would be appropriate for this task.
It is also known to troubleshoot a large circuit board by placing it upon a grid of pointed pins, also referred to as a "bed of nails." Automatic test equipment can select various pins and inject a stimulating current or voltage. Likewise, another pin can be selected to measure the current or voltage at that point. These known systems can be controlled by a computer so that sequences of stimuli and response can be observed, using a switching matrix that allows the sequential tests to occur. See also U.S. Pat. Nos. 3,714,572; 3,842,346; 4,138,643; 4,454,585; 4,342,957; 4,342,958; 4,345,201; 4,377,782; 4,445,085; 4,730,159; and 4,746,861.